1. Field of the Invention
The present invention relates to an apparatus and method for separating particles or components of a biologic fluid, such as blood. The invention has particular advantages in connection with separating blood components, such as white blood cells and platelets.
2. Description of the Related Art
In many different fields, liquids carrying particles must be filtered or processed to obtain either a purified liquid or purified particle end product. In its broadest sense, a filter is any device capable of removing or separating particles from a substance. Thus, the term “filter” as used herein is not limited to a porous media material but includes many different types of devices and processes where particles are either separated from one another or from liquid.
In the medical field, it is often necessary to filter blood. Whole blood consists of various liquid components and particle components. The liquid portion of blood is largely made up of plasma, and the particle components include red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). While these constituents have similar densities, their average density relationship, in order of decreasing density, is as follows: red blood cells, white blood cells, platelets, and plasma. In addition, the particle components are related according to size, in order of decreasing size, as follows: white blood cells, red blood cells, and platelets. Most current purification devices rely on density and size differences or surface chemistry characteristics to separate and/or filter the blood components.
Typically, donated platelets are separated or harvested from other blood components using a centrifuge. White cells or other selected components may also be harvested. The centrifuge rotates a blood separation vessel to separate components within the vessel or reservoir using centrifugal force. In use, blood enters the separation vessel while it is rotating at a very rapid speed and centrifugal force stratifies the blood components, so that particular components may be separately removed. Components are removed through ports arranged within stratified layers of blood components.
White blood cells and platelets in plasma form a medium-density, stratified layer or “buffy coat”. Because typical centrifuge collection processes are unable to consistently and satisfactorily separate white blood cells from platelets in the buffy coat, other processes have been added to improve results. In one procedure, after centrifuging, platelets are passed through a porous woven or non-woven media filter, which may have a modified surface, to remove white blood cells. However, use of the porous filter introduces its own set of problems. Conventional porous filters may be inefficient because they may permanently remove or trap approximately 5-20% of the platelets. These conventional filters may also reduce “platelet viability”, meaning that once the platelets pass through a filter, a percentage of the platelets cease to function properly and may be partially or fully inactivated. In addition, porous filters may cause the release of bradykinin, an inflammation mediator and vasodialator, which may lead to hypotensive episodes in a patient. Porous filters are also expensive and often require additional time-consuming manual labor to perform a filtration process. Although porous filters are effective in removing a substantial number of white blood cells, inactivated platelets may clog the filter. Therefore, the use of at least some porous filters is not feasible in on-line processes.
Another separation process is one known as centrifugal elutriation. This process separates cells suspended in plasma without the use of a membrane filter. The plasma, which carries the cells in suspension, is introduced into a funnel-shaped chamber located on a spinning centrifuge. As additional liquid flows through the chamber, it sweeps smaller sized, slower-sedimenting cells toward an elutriation boundary within the chamber, while larger, faster-sedimenting cells migrate to an area of the chamber having the greatest centrifugal force. This type of chamber, called a leuko-reduction or LRS chamber, is described, for example, in U.S. Pat. No. 5,674,173 and U.S. Pat. No. 6,053,856. It is desirable for an LRS chamber to separate greater than 99.99% of entrained WBC from platelet or plasma products obtained by centrifugal apheresis, which is an extremely high value. The process for cell separation employs a saturated bed that operates in the dense-phase flow regime, which is characterized by high cell density and relatively low fluid flow rate. The primary phenomena that limit LRS performance are Coriolis effects and laminar mixing. Both effects disrupt the desired ideal uniform axial flow in the chamber and the effects have not been completely eliminated in prior configurations.
For these and other reasons, there is a need to improve separation of components of blood in centrifugal blood separators.